Development of the Partial Pressure of a Gas in Solution
Henry’s law explains the behavior of gases in solutions and can be extended to body tissues. In many ways, in-halation anesthetic agents appear to be inert gases that interact with tissues and liquids physically rather than chemically. Therefore, laws governing the physical asso-ciation of gases and liquids are of paramount impor-tance to an understanding of the pharmacokinetics of these drugs. Henry’s law describes the regulation of a gas concentration in a liquid when the association of these two phases is through physical interaction alone. The law states that at equilibrium, the concentration of gas physically dissolved in a liquid is directly propor-tional to the partial pressure (or tension) of the agent and its affinity for the molecules of the liquids (or its solubility in the liquid).
For a clear understanding of Henry’s law, it is im-portant to consider each of its component parts.
Inherent in Henry’s law is the concept that when a liq-uid is exposed to a gas, a partial pressure equilibrium will be achieved between the gas and liquid phases. Thus, molecules of the gas that are physically dissolved in the liquid will exert tension that is equal to the par-tial pressure of the gas above the liquid. It is not neces-sary that a defined gas space, such as a bubble, exist be-fore pressure can be generated. Individual molecules of gas become surrounded and separated by liquid or tis-sue molecules. Furthermore, since they are inert and do not combine chemically with the solvent, the gas mole-cules remain independent and therefore are free to un-dergo random molecular motion and exert pressure equal to that in the gas phase.
Practically speaking, this concept explains the basis for the establishment of partial pressure equilibrium of anesthetic gas between the lung alveoli and the arterial blood. Gas molecules will move across the alveolar membrane until those in the blood, through random molecular motion, exert pressure equal to their coun-terparts in the lung. Similar gas tension equilibria also will be established between the blood and other tissues. For example, gas molecules in the blood will diffuse down a tension gradient into the brain until equal ran-dom molecular motion (equal pressure) occurs in both tissues.
A primary force opposing random molecular motion is the affinity of gas molecules for the tissue in question (a second factor in Henry’s law that expresses the degree of solubility of that agent in the tissue). If a particular gas has a strong affinity for the molecules of a solvent, its random molecular motion will be impeded by a great number of collisions with the solvent molecules. Therefore, it will require a greater volume of an agent of high affinity (or greater solubility) to enter a tissue to generate the same partial pressure as does an agent of low affinity (or lower solubility).
The anesthesiologist can control brain concentration of gas only by modifying the partial pressure of the agent that is delivered to the alveoli. The gas then diffuses across the alveolus to the blood and ultimately into the CNS. The final concentration of gas in the tissue is a function of the partial pressure and the affinity for the tissue (i.e., Henry’s law).